1. Field of the Invention
The present invention relates to a device for gripping flat, thin, workpieces, on an end effector used for handling and transporting such workpieces.
2. Brief Description of Related Developments
Common in the processing of semiconductor substrates and similar devices is the desire to move the substrate from station to station, into and out of various chambers at a given station, and amongst multiple orientations as indicated by the overall processing layout. To accomplish this, the substrate is generally releasably held on a holding platform or end effector as a suitable transport mechanism or robot.
The throughput of any processing system is dependent on the speed at which the substrate can be moved. Therefore each movement in each step of the process has a direct impact on throughput. There is a need to speed up the movements of the end effector at all phases of the process. This has placed increased burdens on the means used to secure the substrate on the end effector platform in order to avoid displacement of the substrate.
There are many types of conventional handling systems specifically designed for holding workpieces like semiconductor wafers, and transporting them between workstations and or wafer cassettes. Among these are passive holding systems using an annular ring member to position a wafer and an annular flange or lip to support the workpieces, for example, in U.S. Pat. No. 4,584,045, U.S. Pat. No. 4,749,330, and U.S. Pat. No. 5,334,257, which are hereby incorporated by reference in their entirety. Many of these conventional types of passive holding systems may not provide sufficient grip for use on high speed robots.
Another conventional approach to increase holding capability is to provide an active grip that uses a vacuum platform, wherein a flat upper surface with vacuum ports is brought into contact with the underside of a workpiece and suction is applied at the port to secure the workpiece to the platform, U.S. Pat. No. 4,770,590 U.S. Pat. Nos. 5,064,030, 5,135,349 and 5,765,444, which are hereby incorporated by reference in their entirety, disclose such a means.
Still another approach to solve these pressing problems makes use of a Bernoulli effect on the top side of the workpiece. U.S. Pat. Nos. 5,080,549 and 5,324,155, which are hereby incorporated by reference in their entirety, disclose such a system wherein a paddle with appropriately placed gas exhaust ports is brought into close proximity with the top side of a workpiece. Nevertheless, the gripping systems employing suction effects for holding the workpieces are complex (employing complex tight fittings at movable joints) and expensive to manufacture. Moreover, such system cannot operate in a vacuum environment.
Still another conventional approach for holding workpieces on transports is the use of one or more movable mechanical grippers such as gripping arms, fingers or tabs. The grippers are moved relative to one another to change the distance between contact surfaces which, when brought to a gripping position, clamp the workpiece in place relative to the transport assembly. In systems of this type it is advantageous to construct the contact surface in a manner that allows engagement of the workpiece at its circumference/outer edge.
Gripping mechanisms that are designed to provide movement of the contacting and clamping members are described in U.S. Pat. Nos. 6,174,011 and 6,322,312, which are hereby incorporated by reference in their entirety. The '011 patent describes a circular disc that supports the substrate. The circumference of the disc is expanded and contracted by the operation of a series of radially oriented leaf springs to allow three circumferentially displaced holding fingers to disengage and engage the substrate. In the '312 patent a pair of clamp fingers are spring biased to engage the substrate from one side to force the substrate into engagement with opposing fixed clamp fingers. The above illustrate the problems with conventional transport apparatus with conventional gripping mechanism. For example, the conventional gripping mechanism described in U.S. Pat. No. 6,174,011, which is hereby incorporated by reference in its entirety, provides a gripper with actuable holding fingers distributed around the perimeter of substrate that results in low force contact between holding fingers and substrate edge during closure of the gripper and transport system movement (i.e., inertial efforts between substrate and holding fingers). However, the gripping mechanism in this case is complex and costly to manufacture (e.g. the flexible radial spokes 40 are expensive and time consuming to cut even when using a wire cutting tool). The examples of the gripping mechanisms described in U.S. Pat. No. 6,322,312, which is hereby incorporated by reference in its entirety, generally include grippers with actuable clamp fingers on but the proximal side (i.e. closest to the base of the end effector) of this substrate (fixed fingers are located on the distal side providing a gripper that is not as effective in holding the substrate at high transport speeds and also suffers from lower gripper closure speeds), and grippers having all clamp fingers that are actuable but with a complex actuation system. The transport apparatus in accordance with the exemplary embodiments of the present invention overcome the problems of the conventional substrate transport apparatus as will be described in greater detail below.